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Electrolytes & Fluids · Clinical background

Hypomagnesemia / MgSO4 CRI

Standard sliding-scale MgSO4 CRI for hypomagnesemia. Magnesium is a cofactor for hundreds of enzymes including Na/K-ATPase, and Mg deficiency is a frequent cause of refractory hypokalemia and hypocalcemia (Mg is required for PTH release). Severe hypomagnesemia can cause refractory ventricular arrhythmias (including torsades de pointes), tetany, and seizures.

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Clinical background

Magnesium is the underrated electrolyte. Most clinicians can recite K and Ca derangements in their sleep but skip Mg, even though Mg deficiency is the reason a non-trivial fraction of refractory hypocalcemia and refractory hypokalemia don’t correct. The clinical signs of hypomagnesemia are also less specific than for K or P, generalized weakness, anorexia, ileus, sometimes ventricular arrhythmias, which means the diagnosis is often made by inference rather than by recognizing a syndrome. The good news: when you do measure Mg and find it low, supplementation is straightforward and the response is usually quick.

Why magnesium matters

Mg is a cofactor for hundreds of enzymes, including:

Mg is also a physiologic calcium-channel blocker, high Mg blocks Ca entry, low Mg disinhibits Ca entry. Clinical relevance: hypomagnesemia can produce ventricular tachyarrhythmias including the classic torsades-de-pointes, and Mg supplementation is itself a treatment for some refractory arrhythmias.

Total vs ionized magnesium

Most clinical labs report total serum magnesium. Ionized Mg is the physiologically active form and is more reliable, but ionized assays are less commonly available. Total Mg correlates reasonably well with ionized Mg in most clinical contexts, with the same caveat that protein-binding affects total values, severely hypoalbuminemic patients can have low total Mg with normal ionized Mg.

The reference range for total Mg varies somewhat by lab and species, but typical values are:

Always check your lab’s reference range, there’s enough between-lab variation that a value at the borderline can be normal at one lab and abnormal at another.

When does hypomagnesemia develop?

A few clinical contexts where hypomagnesemia is common enough to look for:

DKA management. Insulin shifts Mg intracellularly alongside K and P. Hoehne notes Mg decline in both dogs and cats during DKA therapy. The drop is usually less dramatic than the K and P drops but is real.

Chronic loop or thiazide diuretic therapy. Furosemide is the classic culprit in veterinary patients on chronic CHF treatment. Mg is wasted in the urine alongside K. Patients with refractory hypokalemia on chronic furosemide should always have Mg checked.

Refractory hypokalemia of any cause, the workup includes Mg.

Refractory hypocalcemia of any cause, same.

GI losses. Chronic diarrhea, malabsorption, and protein-losing enteropathy can produce significant Mg loss over time.

Refeeding syndrome. Less prominent than the P shift but real. Mg shifts intracellularly during refeeding alongside P.

Primary hyperaldosteronism in cats. Underrecognized cause of hypokalemia, hypomagnesemia, and hypertension in older cats. Workup includes aldosterone:renin ratio and adrenal imaging.

Hyperthyroidism and diabetes mellitus, both can produce mild chronic Mg wasting.

Severe burns, sepsis, pancreatitis, non-specific causes through a mix of GI loss, urinary loss, and intracellular shifting.

The sliding scale

The standard sliding scale by total serum Mg:

Serum Mg (mg/dL) MgSO4 rate (mEq/kg/day) Severity
> 1.5 not indicated normomagnesemia
1.2–1.5 0.25 mild
0.9–1.2 0.5 moderate
< 0.9 1.0 severe

The rate range (0.25–1 mEq/kg/day) is from Hoehne / Box 73.1; the band assignments are conventional veterinary practice and align with DiBartola Ch. 8.

Note that the dosing convention here is mEq/kg/day, not mEq/kg/hr like KCl or KPhos. The hourly rate is correspondingly small, a 20 kg dog at 1 mEq/kg/day with 50% MgSO4 stock infuses about 0.21 mL/hr, or 5 mL over 24 hours.

Stock concentrations

Standard MgSO4 (magnesium sulfate) parenteral solution is 50% weight/volume = 500 mg/mL ≈ 4 mEq/mL. Some hospitals stock 25% (250 mg/mL = 2 mEq/mL) as well. Always read the vial label before drawing, both concentrations look similar in standard amber injection vials.

The InfusionFox calculator defaults to 50% (4 mEq/mL) since that’s the most common veterinary stock; the 25% option is available via toggle on the page.

Administration

MgSO4 must be diluted before IV administration. Direct IV push can cause hypotension, flushing, tachycardia, and (with very rapid administration) cardiovascular collapse. Standard practice:

Compatibility with calcium-containing fluids (LRS, Plasma-Lyte) is generally fine at standard CRI dilutions, but most institutions use D5W or 0.9% NaCl to avoid theoretical concerns about Mg-Ca-P precipitation.

For severe symptomatic hypomagnesemia (refractory ventricular tachycardia, severe seizures), faster bolus dosing has been described, typically 0.1–0.2 mEq/kg over 5–15 minutes, but this is outside the standard sliding scale and should be done with continuous ECG and a clear indication.

Adverse effects to watch

Hypermagnesemia signs in roughly the order they appear:

Reduce or discontinue the infusion if any of these develop and recheck Mg. The therapeutic margin is reasonably wide for a typical sliding-scale CRI but narrows substantially in renal insufficiency. Mg is renally excreted, and patients with reduced GFR can develop hypermagnesemia at standard rates.

In severe hypermagnesemia with cardiovascular compromise, IV calcium gluconate is the immediate antidote, it competes with Mg at the calcium-channel level and can stabilize the membrane while you support the patient.

Monitoring

When to look harder

If a patient’s Mg won’t stay normal despite supplementation:

When you can’t measure Mg

In a patient with refractory hypokalemia who can’t have Mg measured (lab not available, sample issue), empiric Mg supplementation at the moderate-tier rate (0.5 mEq/kg/day for 24 hours) is reasonable and unlikely to cause harm in a patient with normal renal function. If K starts correcting, you’ve diagnosed the deficiency. If it doesn’t, reassess for other causes.

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